This invention relates generally to electronic systems for vehicles and, more particularly, to global positioning systems for vehicles.
The Global Positioning System (GPS) is a satellite-based radio navigation system capable of providing continuous position, velocity, and time information to an unlimited number of users throughout the world. The global positioning system includes a satellite constellation in orbit around the earth. The satellites transmit orbit data. By measuring the ranges from the satellites to a low cost global positioning system receiver, the three-dimensional location of the receiver can be accurately located, provided that the signals from a plurality of satellites, typically four or more satellites, can be received.
Applications of global positioning system in vehicles, such as automobiles, trucks, vans, sport utility vehicles, minivans, and the like, have been developed. Examples of present applications of the global positioning system to vehicles include automatic navigation systems for driver assistance in route guidance, intelligent vehicle highway systems for road tolling and traffic flow assessment and route diversions, as well as automatic vehicle location systems for monitoring a vehicle's position and movement which is provided to a fleet control center.
A global positioning system receiver includes one or more receiving antennas and an electronic system which converts the satellite signals received by the antenna to information pertaining to the location of the vehicle, as well as auxiliary information, such as precise time measurement. One difficulty with application of the global positioning system to vehicles is positioning of the receiving antenna or antennas. In order to precisely receive the signals transmitted by the satellites, a clear line of sight must be provided between the antenna and the satellite without blockage by intervening electrically conductive materials, such as metals. One solution has been to mount a separate casing to the roof of the vehicle with the antenna in the casing. While such solution provides a clear line of sight to the global positioning system satellites in a hemisphere around the antenna, this mounting technique results in a protrusion from the vehicle, which is visually distractive and aerodynamically disturbing. Additionally, means must be provided to attach the casing to the vehicle body, which must either penetrate the vehicle's sheet metal exterior surface or adhere to the exterior surface. Another solution that has been proposed is to form the upper portion of the vehicle passenger compartment roof of a non-conductive material, with the receiving antenna positioned under the electrically non-conductive roof. Such extensive modification to the vehicle would not readily be commercially acceptable to automobile manufacturers and would preclude any simple retrofit of existing vehicles. Locations in most other parts of the vehicle are precluded by extensive use of metal in the vehicle, which results in a cage effect, wherein the areas are significantly surrounded by electrically conductive metal.
Accordingly, a need exists to find a manner of mounting a receiving antenna of a global positioning system receiver that would hide the antenna from observation in order to avoid distracting from the appearance of the vehicle and to avoid tipping off thieves to the presence of the receiving antenna and the global positioning system receiver. Such mounting technique should additionally provide for ease of accommodation of the receiving antenna without significant modification to the vehicle or its internal structure. Furthermore, the mounting must be achieved in a manner which accommodates the functionality of the receiving antenna by providing a clear line of sight, free from any cage effect, to a sufficient number of transmitting global positioning system satellites in order to carry out the purpose of the application.